The invention relates to an arrangement for the conversion of an electrical input quantity into an electrical DC signal proportional thereto, comprising a digital pulse duration modulator having a limited resolution, which provides a periodic pulse sequence which is pulse duration modulated with the quantization value, corresponding to the limited resolution, of the input quantity neglecting the quantization remainder, and an averaging circuit for forming the DC signal mean value of the pulse duration modulated pulses.
In arrangements of this type the accuracy with which the electrical input quantity is converted into the DC signal proportional thereto, is limited to the accuracy corresponding to the resolution of the digital pulse duration modulator.
If for instance a commercially available 8 bit pulse duration modulator is used, the total value range of the input quantity may only be converted into 2.sup.8 =256 quantization values and accordingly the DC signal may only assume respectively one of the 256 values proportional thereto. Intermediate values of the input quantity, which correspond to a quantization remainder differing from zero, are represented by the next lower value of the DC signal. The accuracy obtainable in this manner is insufficient for many applications.
One object of the present invention is to provide an arrangement of the before-mentioned type which, while adhering to the principle of conversion by digital pulse duration modulation with a limited resolution, leads to enhanced accuracy.
In order to achieve this object the arrangement in accordance with the invention comprises a first pulse duration modulator of a limited resolution which supplies a first periodic pulse sequence which is pulse duration modulated with a quantization value, corresponding to the limited resolution, of the input quantity neglecting the quantization remainder, a second digital pulse duration modulator of limited resolution which supplies a second periodic pulse sequence which is pulse duration modulated with the sum of said quantization value and of the quantization remainder multiplied by a predetermined factor, means for forming a combined periodic pulse sequence by selectively transmitting pulses of said first and second periodic pulse sequences in such a manner that the ratio of the number of transmitted pulses of said second periodic pulse sequence to the total number of transmitted pulses in said combined periodic pulse sequence is equal to the reciprocal of said predetermined factor, and an averaging circuit which receives said combined periodic pulse sequence and forms the DC signal mean value thereof.
The enhancement of the accuracy obtained in accordance with the invention is due firstly to the fact that the quantization remainder, which remains out of consideration during the first pulse duration modulation, is transformed by the multiplication with the predetermined factor to a value which can be used with the limited resolution for the pulse duration modulation, and secondly that this enhanced value is then distributed by the pulse selection on average to a number, corresponding to the factor, of pulse periods so that for each pulse period there is a fraction which corresponds to the real quantization remainder. The enhancement of the accuracy corresponds to the resolution with which the multiplied quantization remainder is expressed. If for instance the quantization remainder is represented with a resolution of 3 bits, there will be an enhancement of accuracy by a factor of 2.sup.3 =8, and the conversion obtained with two 8 bit pulse duration modulators will be equivalent to a conversion obtained with an 11 bit pulse duration modulator.